Vertex vs. Fitting

[Judy Canty]

I know I've asked this question before, but I don't think I got an answer.

The "vertex distance" we're asking ECP's to take is what I've always known as the "fitting distance". Vertex distance was the measurement that a prescriber noted as the distance from the front surface of the cornea to the phoroptor. The compensated power was determined by the difference between the two.

So, of what actual use is the "vertex distance" we're now asking ECP's to provide when ordering "individualized" PAL's?

[Darryl Meister]

This is indeed a rather confusing and often misunderstood topic. Typically, there is the refracted vertex distance used by the clinician during the subjective refraction procedure and the fitted vertex distance of the actual spectacle lens on the wearer. And, as you noted, you would compensate for any difference between the two in stronger prescriptions using a common formula.

Lens designers also rely on the fitted vertex distance of the actual spectacle lens, since ray tracing calculations depend upon the distance from the back of the spectacle lens to the center of rotation of the eye, known as the stop distance. Note that these calculations are independent of any vertex distance compensation that was applied as a result of the difference between the refracted vertex distance and the fitted vertex distance.

If only one vertex distance is requested for a free-form lens, it is typically the fitted vertex distance of the spectacle lens, which will be used in the ray tracing calculations. In this case, if the prescription is strong and there is also a significant difference between the refracted vertex distance and the fitted vertex distance, you would still want to compensate the prescription for the vertex distance, before submitting the prescription to the laboratory.

Best regards,
Darryl

[Judy Canty]

Thank you Darryl!

[loose-screw]

I have sat through several seminars on digital lenses. The most recent one was from Shamir.
The vertex distance is exactly what Darryl said.
What I have learned in the seminars has been that most "Digital" lenses need a Vert distance but they can default them. I believe that number is 13 mm if there is a bigger difference + or - it will compromise the compensated Rx. Shamir has given my lab several Nifty tools that measure face form vert distance and pantoscopic tilt.
Now I will also say we tell all of our accounts about the vert distance but I can't recall any of them specifying one, and the only time I have truly had any need for one has been a high plus progressive wearer using a high wrap frame.

[Robert Martellaro]

My optical software (vintage 1980's) uses doctor's Vertex (DVtx) and wearer's vertex (WVtx).



I don't believe any the manufacturers that optimize for POW take the doctor's/refracted vertex and make the necessary compensation.

[chip anderson]

Until an Rx gets really strong, in spectacles the difference between worn and Rx's is not too signifcant. Especially when you adjust the nose pads from where the lab screwed them up to put the lenses in and the patient re-positions them to where he is going to wear them on his face no matter what you had in mind. Now Spectacle Rx to contacts where the difference and be 11 to 18mm can make quite a difference, but even then it only begins to matter after 4 diopters of power. Don't worry too much about it, but don't completely forget it either.

Chip

[Barry Santini]

Ah, but VD really does matter in astigmatic pals, because it defines the area weighting a design might use, depending on how sophisticated it is, and what its goals are

[Darryl Meister]

The fitted vertex always matters in modern progressive lens design, but the refracted vertex only matters to the extent that the distance refraction has not been corrected for the fitted vertex distance.

The problem gets a little more complicated than this for the near refraction, but the small differences involved are generally ignored.

Best regards,
Darryl